A conventional vehicle radar device radiates transmission waves such as light waves or millimeter waves toward the front of the vehicle and detects objects in front of the vehicle based on the reflected waves as disclosed in, for example, JP-A-2002-40139. The device of this kind is used for a device that generates an alarm upon detecting a distance to the preceding vehicle or a device which controls the vehicle speed so as to maintain a predetermined distance to the preceding vehicle.
In the above vehicle radar device, the direction of radiating the laser beam emitted from a laser diode is varied by using a polygonal mirror that is rotated, thereby to radiate a plurality of laser beams over a predetermined angular range in the direction of width and height of the vehicle. When the laser beams are reflected by objects, the reflected beams are received through a light-receiving lens. The reflected beams that are received are guided to a light-receiving element which then produces voltage signals corresponding to the intensities of the received beams.
The distance to the reflecting object is detected based on a time interval up to when the voltage signal becomes greater than a reference voltage from when the laser beam is radiated, and a position, too, is detected in the direction of vehicle width and in the direction of vehicle height based on the angle of irradiation of the laser beams.
As described above, the objects to be detected by the vehicle radar device are the preceding vehicles, etc. that are present ahead. Usually, a preceding vehicle has reflectors on the rear surface thereof to reflect the transmission waves such as laser beams, and the vehicle body, too, has a relatively high reflection intensity though it is not as high as that of the reflectors. Therefore, the vehicle radar device is capable of detecting the preceding vehicles that are traveling ahead to maintain distances of not shorter than 100 meters.
However, the output intensity of the transmission waves such as laser beams is limited due to a variety of reasons, and a limitation is inevitably imposed on the detectable range of distance to the preceding vehicles.
Besides, when mud, snow, etc. adhere to the rear surfaces of the preceding vehicles, the intensity of laser beams reflected by the preceding vehicles decreases. When the light signals are received in this case, it becomes difficult to distinguish noise components generated due to various factors over the received light signal components having intensities corresponding to the light reflected by the preceding vehicles. As a result, the detectable distance of the radar device becomes short.